RFC 2396 defines the terms universal resource identifier (URI) and Resource. A URI reference may contain a URI and a fragment identifier but what is identified by a URI reference remains unclear.
When dereferenced a URI returns an entity and the particular entity may vary over time, or conditions of the network request. A URI is mapped to a single Resource and a set of Entities.
An entity resolved over a network is returned as a character stream. It may be associated with a MIME media type which defines a format. Formats are often formally described using EBNF production rules.
Entities that conform to a particular format or schema are termed to be instances of the format or schema. The EBNF production rules define an abstract syntax and set of nodes in which the entity parse tree is represented. In the default case, a character stream is represented by a single root node and a series of child nodes representing each character in the stream.
Nodes may be addressed by appropriate identifiers. A fragment identifier is mapped to a node within a parsed entity. Generally a URI reference is bound to an abstract node which corresponds to a set of nodes each a member of one of the set of entities bound to the URI.